The production of coated small particles for use in the drug industry is widely known. Typically fluidized bed systems, such as the Wurster process coater have been used. Hall, H. S. & Pondell, R. E., "The Wurster Process" in Controlled Release Technologies: Methods, Theory & Applications, Vol. II. A. Kydonieus, ed. CRC Press, Boca Raton, FL 1980, pp. 137-138. In the fluidized bed coating chamber, the solids to be coated are suspended in a heated stream of air. The coating material is sprayed onto the suspended solids until the desired amount of coating has been applied. The coating solution is sprayed slowly over a period of time, so that the fluidized particle bed does not become wet and sticky, resulting in agglomeration. The problem of agglomeration becomes more serious as the size of the core particles decreases, and is particularly acute when coating particles smaller than about 300 .mu.m. The necessity of applying the coating slowly means that each particle receives its coating in many incremental steps as the particle moves through the bed from a region near the coating nozzle to a distant region and back, continuing the cycle throughout the process.
Efforts have been made to replace the slow, incremental coating method with techniques which can produce a coated solid particle in a single rapid step. One technique which has been used is to spray-dry the core material in a film-forming solution of the coating material. The use of ordinary spray-drying atomizers for the process necessitates the use of very small core particles (A. F. Asker and C. H. Becker, J. Pharm. Sci. 55(1), 90 (1966)). In fact, the difficulty of using this process to coat preformed solid particles is such that most workers have chosen instead to utilize a co-solution of core and coating material. (ibid.; H. Takenaka, Y. Kawashima, and S-Y. Lin, J. Pharm. Sci. 69(12), 1388 (1980); Senatore, U.S. Pat. No. 3,521,370.)
Although these fluid bed coaters and spray dryers are effective for many applications, there is a continuing search for improved coating apparatuses.